OK; if you go over the 25GB amount (which only applied M-F 8am-midnight - all other times are not counted towards your 25GB (eg all weekend and midnight to 8am are 'free' on Home package)), you will be charged at £1.10 + VAT per Gigabyte or per part off; so If you used 25.1GB, you would be charged an extra £1.10+VAT that month. You can monitor your usages at http://my.xilo.net - which I see you have bookmarked already (no I'm not spying on you; it's in your screenshot!).
With regards to changing package; there is a £11 + VAT regrade charge; unless you wanted to go to the TalkSurf package were Matt has previously said that there is £0 charge to move from another ISP; not sure about an internal migration though.
Phone is handled by Xilo too; they bill you, they arrange the full migration - you don't deal with anyone other than the guys (and gals) at Xilo - everything goes through them.
Call costs in pence per minute (ex VAT) as follows:
Code:
Daytime Evening Weekend
National Fixed Line 1 0.8 0.8
Local A 1 0.8 0.8
Local B 1 0.8 0.8
Vodafone Mobile 8.5 8.5 8.5
O2 Mobile 8.5 8.5 8.5
Orange Mobile 8.5 8.5 8.5
T-Mobile 8.5 8.5 8.5
Three Mobile 13 8.5 4.8
Opal Mobile 13 8.5 6.5
BT Mobile 13 8.5 7
Dolphin Mobile 13 13 5
Services National 1.5 1.5 1.5